I liked the Heliopan polarizers so much, that I purchased the sh mpc for all my lenses replacing many other much less expensive although well regarded filters. These filters have 16 layer multi-coatings and have produced consistant results with all my camera systems.
If you decide to go with threaded filters you will need a bay adapter, larger filter, and a snap lens cover as the Hasselblad bay lens cover won't work.
It is very hard to argue against B+W (of which I have one) and Hasselblad (of which I have a couple) and Heliopan (of which I have none)...
However, if money or opportunity is an issue, then look at the best end of Hoya... They make high quality multi-coated filters along with their not so high quality filters. I think that HMC as a minimum, but the Super HMC and Pro lines are beyond them and easily have the stats of the others.
The issue of non-brass rings (as in Hoya) has been raised, but I haven't ever found a problem particularly. I do have one cheap Bay 60 that came with a lens and it was too tight, but a few seconds carefully adjusting the bayonet fitting and bob's your uncle!